This invention relates in general to electrical termination systems, and more particularly to an electrical termination device that additionally provides temperature sensing and a temperature indication signal.
Electrical termination systems are commonly used for providing electrical termination to electrical signals present in electrical circuits. Typically, an electrical termination system attempts to provide a predefined impedance or load, such as a fifty ohm impedance, between a signal line and a reference line, such as a ground reference, in an electrical circuit. Ideally, a termination system should provide a constant predefined impedance for a wide range of electrical signal frequencies. Examples of termination systems include, but are not limited to, radio frequency (RF) loads for frequency circulators, couplers, power combiners, absorptive filters, and antenna replacement dummy loads.
Temperature sensing terminations are generally utilized in feedback systems used for fault detection. When a high power termination sees high power, normally this is an indication that something in the system is broken. Therefore, it is generally useful to detect when high power is dissipated in the termination and either remove the power source or throttle it back by some measure to prevent other system damage from occurring.
The most common application of a feedback system is one in which a signal is normally not present until something is broken. The sensor is used to detect a sudden increase in power level from some low level leakage value. For example, in a conventional 100 watt system, the power dissipated in a termination in normal operation would be typically less than one watt, or less than one percent of its power rating. If something then fails in the system, there is a sudden step increase in power from that one percent to nearly 100 percent. Typical applications would include use of a termination on a circulator to terminate reverse power or use of a termination on a coupler in between balanced amplifier stages where no power would be present until the amplifier stages became imbalanced. Less common applications include systems where terminations are used in the presence of some ambient power that needs to be monitored for changes or, in some cases, for signal removal, indicating the end of a fault condition.
There are two basic approaches utilized in the conventional technology. These are RF detection and thermal detection. Regarding RF detection, one approach is to take a coupling device and, sampling a small portion of the RF signal going into the termination, feed the sample to a low power detector such as a diode or semiconductor detector which detects the fault condition. Another approach would be to use a very high value attenuator in place of the termination. The attenuator would not only terminate the RF signal but also sample a portion of that signal to be sent to a low power detector such as a diode or semiconductor detector which would detect the fault condition. There are disadvantages to the conventional approaches of RF detection. By definition, there is RF going into and out of the sensing device so that it is necessary to maintain good RF design principles when taking the sample signal and transmitting it to the sensor. The sample is a low level signal in the presence of a high level signal somewhere in the system. Therefore, there is a chance for crosstalk or false alarming if the two signals do not remain separated. Also, the use of a semiconductor device is a relatively high cost approach which requires a number of components and is relatively complex to design. Yet another disadvantage, in regard to the use of a high power RF attenuator where more than one resistor element is present, the junctions between the resistor elements form unwanted parasitic reactance in the design, which makes it more difficult to achieve a low VSWR, voltage standing wave ratio.
Another conventional RF approach is to use a low power thermistor termination. The thermistor provides both the termination and temperature sensing features. RF power is driven directly into the thermistor device itself to self-heat the thermistor and change its power level. This low power, self-heating device is used in place of a diode or semiconductor-type detector. A disadvantage is that it is strictly for low power applications. It cannot be used directly as the termination in a high power system. A coupler or high value attenuator would be required to support it.
Another disadvantage is, because the RF signal is passing through the thermistor, it is necessary to filter away the AC signal to leave the remaining DC information. The fact that the RF passes through the thermistor, which has a nominal impedance of about 50 ohms, means that the source impedance of the sensor is nearly 50 ohms. The lower the impedance of the sensor, the more difficult it is to filter the RF. Yet another disadvantage is, since all of the sensing is done in a device, it is difficult to separate the ambient temperature from the temperature derived from dissipating power.
In regard to thermal detection, external variable resistant devices or thermistors are glued or mechanically attached to a high power termination and used to detect when the entire body of the device heats up. There are disadvantages to this approach also. One disadvantage is that the external device is not tightly coupled to the termination resistor element so there may be a significant time lag between when the temperature dissipation occurs in the resistor element and when the sensor detects the corresponding temperature. Also, the presence of the sensor in close proximity to the electrical field generated by the RF termination will, in many cases, de-tune the RF characteristics that were designed into the termination device.
Thus, there exists a need for a temperature sensing termination system that does not have the disadvantages of the prior art.